Acoustical plaster and method of making



6 8 6 EXAMllltR CROSS REFERENCE Patented Oct. 18, 1949 ACOUSTICAL PLASTER AND METHOD OF MAKING Willard L. Chrisler, Kenmore, N. Y., assignor to National Gypsum Company, Buffalo, N. Y.

No Drawing. Application November 29, 1946, Serial No. 713,081

2 Claims. (01. 106-114) The present invention relates to acoustical plasters and more particularly to acoustical plasters possessing improved water retention during working under the trowel and characterized by ease of application of the plastic mix thereof is far superior to acoustical plasters heretofore available.

It is another object of the invention to dean increased time of working. 5 crease the loss of water from plastic mixes of Heretofore the acoustical properties of rooms acoustical plaster'to the base to which it is apand auditoriums have been improved by treatplied and by evaporation to the air. ing the walls and ceilings thereof with porous It is also an object of the invention to increase sound absorbing materials. Such materials are the length of the working period of the plastic either of an inorganic nature or porous, fibrous mix of acoustical plaster under the trowel, materials. Such acoustical correction agents Other objects 0 the invention are to increase were applied to the walls or ceilings or both of the strength of t e set acoustical plaster and to the space, the acoustical properties of which was increase the plasticity of the mix of the acousto be corrected, either by attaching panels of the tical plaster and water. acoustical correction agent thereto or by apply- 5 In accordance with the present invention an ing such agent as a continuous film or cover improved dry acoustical plaster mix is prepared coat in plastic form which subsequently set or from a porous aggregate, an inorganic water sethardened. ting cementitious material and a small amount Where the acoustical correction agent was atof a natural gum which aids in retaining the tached as a panel it was normally either bonded gaging water in the plastic mix when applied to the walls and ceilings by a cement or bondand in increasing the working time of such mix. ing agent or by means of intricate systems of clips As the efficiency of the acoustical plaster depends and hangers. Acoustical plasters, as used heretoupon the number and size of intercommunicatfore, comprised a porous aggregate and a cemening pores whereby sound waves striking theretitious bonding agent capable of being rendered against are absorbed and their energy dissipatplastic by the addition of water such as Portland ed within the body of the plaster rather than .cement or calcined gypsum or lime. I reflected therefrom, the aggregate comprises Since acoustical plasters are normally applied crushed, highly porous, inorganic materials such were base coat of plaster suchacoustical correcas pumice, r h m d th l k to hi h tion agents were normally applied heretofore by may be added asbestos fibres as well as organic plasterers accustomed to work with ordinary fibrous materials such as wood fibres, waste wall plasters. However, due'to the diiference in paper pulp, and the like. In general, the amount nature between ordinary wall plasters and acousof aggregate is considerably in excess of the tical plasters, difficulty has been encountered in amount of inorganic cementitious bonding app y g t latter p y to a Wall 0611- agent. The bonding agent is generally plaster ing. This difficulty is traceable to two main difof Paris although Portland cement, or other ceferences between plastic mixes of wall plasters mentitious material capable of setting in the and plastic mixes of acoustical plasters. In the resence'of water may be employed. first place, due to the presence of the extremely The acoustical plaster may also contain in adporous aggregate, acoustical plasters lose water 40 dition to the principal constituents, namely, the y absorption i o the y base o which y ag regate and the water-setting bonding agent re ppli d an also by evaporation i the ai or cement, a number of other materials which to a much greater extent than wall plasters and, improve various characteristics associated with secondly, acoustical plasters acquire an initial the acoustical properties or the application of set much more rapidly than wall plasters and, 5 the plaster to the wall. Thus, finely-divided ashe a tr me y d flicult t fl at a d d y bestos fibres tend to improve the plasticity and tothc proper leve d t t r slip of the plastic mix obtained by addition of Due to the porous nature of the aggregate the water to the plaster-aggregate combination. bonding agent in the acoustical plaster may tend Cellulose or wood fibres tend to improve the bulkt d y ut y as n f water l s prior t s ing properties of the plaster since they aid in a d fo a c a y a d W ak d resulting in a water-absorption and help in providing cohe- ,friable coat of plaster with insufiicient inherent sion of the wet mix and thus prevent settling strength. of the aggregate and non-uniformity within the It is an object of the present invention to promix. vide an improved' acoustical plaster wherein the It is also customary to include within the mix agents which control the setting time of the cementitious bond when in contact with water. Such agents as are added normally tend to retard the settin time and thus permit better working of the mix under the trowel. Such substances are well known in the trade and the usual W3 or other alkali digested protein su tances. ditionally gas entraining agents which tend to increase the bulk of the wet mix and which impart, in part at least, a portion of the cellular structure of the finished plaster so necessary for effective sound absorption, may be included.

By adding natural gums to the acoustical plaster mix in accordance with the present invention, such as locust bean gum or gum ghatti, in small amount on the order of 0.1% to 0.5% of the total weight of'the dry acoustical plaster, the time available in which a water mix of the plaster may be troweled is greatly increased and, in addition, the loss of water to the plaster supporting wall and by evaporation to the air is also greatly diminished. Acoustical plasters made in accordance with the present invention and in which, therefore, a small amount of gum ghatti or locust bean gum has been incorporated provides from three to four times the usual working time of the same acoustical plaster without the inclusion of these natural gums.

As the water retention and working time of the plaster have been improved, the acoustical plaster, after application and set, possesses greater strength since any tendency to chalk on the exposed edges due to rapid loss of water has been greatly diminished.

Strange as it may seem, benefits or this character are not imparted when employing gum tragacanth, algin, or karaya gum and other related natural gums. Acoustical plaster mixes to which such natural gums have been added tend to increase the water-carrying capacity slightly but do not appreciably improve the water retention of the wet mix when applied and worked over a dry base. Of all the natural gums only gum ghatti and locust bean gum appear to possess the desired property of both increasing watercarrying capacity to a considerable degree and of increasing the time of working of the mix under the trowel whereby the mix may be floated or darbied to the desired level and texture by the plasterer.

The following example is given merely by way of illustration and is not to be deemed limitative 4 of the invention since it will be understood that the amounts and relation of aggregate and bonding agent may vary over a considerable range, the formulation given below being intended to be merely a typical formulation of a dry acoustica Plaster mix.

Material: Parts by weight Crushed graded pumice 480.00 Plaster of Paris 225.00 Asbestos fibre 60.00 Wood fibre 18.00 Soap (or other gas entraining material) 0.25 Commercial retarder 1.25 Locust bean gum (or gum ghatti) 1.25

The dry acoustical plaster of the formulation given immediately above may be gaged with water in the usual amount to roduce a wet mix of the proper consistency for troweling and will remain plastic and workable for about 3 minutes as contrasted with the same formulation without the presence of the gum, which is workable for only /4 minute. The amount of gum used should be at least 0.1% by weight of the dry mix in order to obtain the desired result and may be present up to about 0.5%; amounts greater than 0.5% do not appear to improve either the water retention or time of working and merely serve to increase the expense of manufacture.

What is claimed is:

1. The method of increasing the working range of an acoustical plaster containing a porous aggregate and a water-setting calcined gypsum which comprises adding thereto at least about 0.1% of a gum selected from the group of natural gums consisting of locust bean and ghatti based on the weight of the dry mix.

2. An acoustical plaster comprising a watersetting calcined gypsum, porous aggregate and at least about 0.1% of a gum selected from the group of natural gums consisting of locust bean and ghatti based on the weight of the dry mix.

WILLARD L. CHRISLER.

REFERENCES CITED The following references are of record in the file of this patent: 

